1. Field of the Invention
The present invention relates to a cathode for an electron tube used in a cathode-ray tube or the like.
2. Description of the Related Art
FIG. 3 shows a conventional cathode for an electron tube disclosed in the Japanese Laid-Open Patent Publication 257735/1991. In the drawing, reference numeral 1 is a base (substrate) composed of a material, in which main component is nickel and a very small amount of reducing elements such as silicon (Si) and magnesium (Mg) is contained. Numeral 5 is an electron emission material layer, in which main component is an alkaline-earth metal oxide 11 containing barium and strontium or/and calcium, and a rare-earth metal oxide 12 such as scandium oxide of 0.1 to 20 weight % is contained. Numeral 2 is a cathode sleeve composed of nichrome and so on. Numeral 3 is a heater placed in the base 1 and emits thermions from the electron emission material layer 5.
Described below is a method for manufacturing the cathode for electron tube composed as described above as well as properties thereof. First, a reducing metal such as tungsten is formed on the upper face of the base so that thickness may be approximately 1 xcexcm through vacuum deposition or the like. Next, ternary carbonate of barium, strontium, and calcium and a predetermined amount of scandium oxide are mixed with a binder and a solvent in order to prepare a suspension. This suspension is applied on the base 1 to be approximately 80 xcexcm in thickness through spraying. After that, they are heated by the heater 3 in a vacuum evacuation process of a cathode-ray tube, and the carbonate is turned into oxide. After that, in a process called an activation process, a part of the alkaline-earth metal oxide is reduced and a free barium to be an electron emission source is formed due to reduction effect of said metal layer and a very small amount of reducing agent in the gas.
In this process, a part of the alkaline-earth metal oxide reacts as described below, and the free barium is generated. The reducing agent such as silicon and magnesium contained in the base 1 moves to the interface between the electron radiation material layer 5 and the base 1 due to diffusion, and reacts with the alkaline-earth metal oxide. For example, in the case where the alkaline-earth metal oxide is a barium oxide (BaO), a free barium generation reaction shown by the following expressions 1, 2 takes place:
2BaO+xc2xdSi=Ba+xc2xdBa2SiO4xe2x80x83xe2x80x83(1)
BaO+Mg=Ba+MgOxe2x80x83xe2x80x83(2)
The barium oxide is reduced at the interface between the metal layer 4 and the electron radiation material layer 5 due to the reduction effect of tungsten, and the free barium is generated in the same manner.
2BaO+⅓W=Ba+⅓Ba3WO6xe2x80x83xe2x80x83(3)
A scandium oxide 12 is added into the electron emission material layer 5 in order to prevent formation of an intermediate layer caused by barium silicate (2Ba2SiO4), magnesium oxide (MgO), barium tungstate (Ba3WO6), and so on generated in the foregoing expressions (1) to (3). This intermediate layer is formed at the interface between the electron emission material layer and the base and obstructs diffusion of the reducing agent.
Moreover, in the conventional cathode for electron tube, the metal layer composed of tungsten is formed on the base in order to generate the free barium as shown in the foregoing expression (3). The metal layer is formed at most 2 xcexcm in thickness because the metal layer of at most 2 xcexcm in thickness does not prevent reducing elements in the gas from diffusing into the electron emission material.
FIG. 4 shows an example of an electron gun for a cathode-ray tube in which the cathode for electron tube obtained as described above is used. In the drawing, numeral 6 is a control electrode, numeral 7 is an accelerating electrode, numeral 8 is a focusing electrode, numeral 9 is a high-voltage electrode, and numeral 20 is a cathode for an electron tube. In an ordinary television set or a display set, a voltage applied to the control electrode 6, accelerating electrode 7, focusing electrode 8, and high-voltage electrode 9 is fixed. Amount of electrons emitted from the electron tube cathode 20, i.e., cathode current, are controlled by modulating the voltage applied to the electron tube cathode 20 itself. For example, establishing the voltage of the control electrode 6 as standard, a voltage from 0 V to cutoff voltage is applied to the electron tube cathode 20. A voltage of plus some hundreds-volt is applied to the accelerating electrode 7. The voltage of the electron tube cathode 20 is adjusted to be near the voltage of the control electrode 6, whereby an electric field from the accelerating electrode 7 consequently permeates through an electron passage hole of the control electrode 6, and electrons are emitted toward a panel for display. The focusing electrode 8 and the high-voltage electrode 9 are arranged to focus and accelerate the electrons emitted from the electron tube cathode 20.
The mentioned cutoff voltage is one of the characteristics of a cathode-ray tube. The cutoff voltage is defined herein as xe2x80x9ca cathode voltage at the boundary of the beginning of electron emission from the cathode under the condition of fixing the voltage excluding the voltage of the cathodexe2x80x9d. This cutoff voltage is generally determined due to the three elements of cathode, control electrode, and accelerating electrode, and depends on the space between each of the electrodes, electrode thickness, and configuration of the electron passage hole. The cutoff voltage is set to be within a predetermined voltage range corresponding to the type of electron gun. However, in the electron tube cathode having tungsten metal as described above, tungsten and nickel which is the main component of the base diffuse mutually during operation. Plastic deformation due to cubical expansion in alloy formation and plastic deformation due to yield of the base metal caused by repeatedly heating and cooling the cathode take place. It is acknowledged that the deformation is increased especially when the metal layer is formed on the whole base. It is known that the electron radiation material layer itself shrinks due to evaporation, sintering, and so on during a long-term operation. Both of the mentioned deformation and shrinkage cause a change in the space with the passage of time between the cathode and the control electrode, i.e., a change in the cutoff voltage with the passage of time.
Described below is influence in the case where the cutoff voltage changes. Change in brightness, i.e., luminance of a cathode-ray tube is mainly caused by decrease in transmission of visible radiation of the panel glass, decrease in luminous efficiency of the fluorescent substance, and decrease in current from the cathode. In particular, considering the decrease in current from the cathode, following two factors are raised. The first factor is that the current value decreases due to deterioration in the ability itself of emitting electrons from the cathode. The second factor is a change in the electric field on the surface of the cathode due to variation in the cutoff voltage. Both of the two factors result in brightness changes.
The present invention was made to resolve the above-discussed problems and has an object of providing a cathode for electron tube capable of achieving a cathode-ray tube for a display in which brightness change is small even when cutoff voltage of the electron tube cathode varies during a long-term operation.
A cathode for an electron tube according to the invention, which decreases change in cutoff voltage during a long-term operation by limiting thickness and void ratio of a metal layer formed on a base and decreasing deformation of a metal of the base, the cathode comprising, the base of which main component is nickel and which contains at least one kind of reducing agent, the metal layer formed on said base, and an electron emission material layer which is formed on said metal layer and of which main component is an alkaline-earth metal oxide containing barium, wherein a porous metal layer is used as said metal layer.
It may be preferable that, in the cathode for electron tube according to the invention, the porous metal layer is not more than 80 xcexcm in thickness and 20 to 70% in void ratio.
It may also be preferable that, in the cathode for electron tube according to the invention, the porous metal layer is formed by the steps of forming a mixture of metal with a vacancy agent on the base, heating the mixture in vacuum or in a reducing atmosphere, to remove the vacancy agent.
It may also be preferable that, in the cathode for electron tube according to the invention, a temperature of 800 to 1100xc2x0 C. is applied to the mixture at said heating step.
It may also be preferable that, in the cathode for electron tube according to the invention, the vacancy agent is composed of thermoplastic resin.
It may also be preferable that, in the cathode for electron tube according to the invention, the thermoplastic resin is methacrylate compound.
It may also be preferable that, in the cathode for electron tube according to the invention, the methacrylate compound is polymethyl methacrylate (PMMA).
It may also be preferable that, in the cathode for electron tube according to the invention, the porous metal layer is 5 to 50 xcexcm in thickness.
It may also be preferable that, in the cathode for electron tube according to the invention, main component of the porous metal layer is a metal selected from the group consisting of tungsten, nickel, silicon, magnesium, zirconium, and aluminum.